1. Field of the Invention
The present invention relates to semiconductor capacitors, and more particularly to a method for forming a cylindrical capacitor of a dynamic random access memory cell.
2. Description of the Prior Art
Demand for dynamic random access memories (DRAMs) has rapidly increased owing to widespread use of integrated circuits. Each cell of the DRAM includes transistors and a capacitor, which is used for the purpose of charge storage. As the DRAM becomes highly integrated, the area occupied by the capacitor of the DRAM storage cell shrinks, thus decreasing the capacitance of the capacitor owing to its smaller electrode surface area. In order to reduce the cell dimension and yet obtain a high capacitance, the cylinder-shaped capacitor, which includes an inner surface in addition to an outer surface, was disclosed to increase the surface area of the capacitor electrode.
Unfortunately, the conventional cylindrical capacitor suffers complex and complicated processing steps, and generates DRAM cells with low quality. A conventional process for forming a cylindrical capacitor includes firstly providing a substrate having transistors manufactured thereon, followed by forming a dielectric layer having contact hole therein. Subsequently, a polysilicon layer is deposited on the dielectric layer and also fills the contact holes. Based on this polysilicon layer, a combination of oxide layer formation, etching stop layer formation, polysilicon layer formation, planarization technique, photolithography and etch process is repeatedly performed in order to finally form a cylindrical structure as shown in FIG. 1A. This resultant structure consists of a silicon oxide pillar 01 surrounded by polysilicon 02. After removing the polysilicon 02, a cylindrical polysilicon electrode is thus achieved as shown in FIG. 1B. Due to the complex and complicated processing steps, the fabricated DRAM capacitors are apt to suffer defects. More specifically, a silicon nitride layer is required to provide as an etch stop while removing unwanted polysilicon. The silicon nitride layer further requires planarization process such as chemical mechanical polishing (CMP). Moreover, as the cylindrical polysilicon electrode takes shape in a number of steps, the joints inside the cylindrical polysilicon electrode are not strong enough to resist pressure, thus decreasing manufacturing throughput and product quality.
FIGS. 2A and 2B demonstrate another conventional method for forming a cylindrical capacitor. In this method, a spacer 04 composed of silicon oxide or silicon nitride is formed over a polysilicon layer 03. After etching the polysilicon layer 03 using the spacer 04 as a mask, a resultant cylindrical capacitor electrode is achieved. This method disadvantageously requires deposition and etching back for forming the spacer. Furthermore, the width of the spacer is not practically controllable in a predetermined manner, thereby usually resulting in a cylindrical capacitor having horn formation on top of the capacitor, which is apt to be broken into fragments, thus complicating the subsequent process.
For the foregoing reasons, there is a need for a method of forming a cylindrical capacitor of a dynamic random access memory cell, so that the process can be simplified, and the manufacturing throughput and product quality can be increased.